Github PR context provider

[StructByLightning]

Validations

• [X] I believe this is a way to improve. I'll try to join the Continue Discord for questions
• [X] I'm not able to find an open issue that requests the same enhancement

Problem

I would like to use Continue to help explain and review Github pull requests during code review. Since a PR often has many small changes spread across numerous files, it's currently impractical to use Continue since I'd have to manually copy in each piece of changed code.

Solution

Add a context provider that inserts the diff data between the current branch and the branch the PR is merging into. The resulting prompt should include the full diffed file, so that way the AI has access to the context that the changes were made in.